Star Trek: 10 Devices That Every Starfleet Ship Has
8. Sensors
The ship's sensors perform a variety of tasks such as star-mapping for navigation, finding a transporter lock, and even detecting someone in front of a closed door to allow them through.
The internal sensors aboard the ship are used for security and can tell the location of any crewmember, as well as any weapons that may go off. The external sensors are the eyes of the ship and can tell the crew how many people are on a planet or aboard a vessel and even make out a single individual among billions of others.
They aren't just used for detecting life signs, though. A thorough scan of an attacking vessel can tell the crew whether weapons are charging, as well as the locations of all ship's systems, allowing for a far more targeted defense.
The sensor's keep a constant log stored in the ship's computer. Nothing happens in or around the ship without it being detected and recorded.